常见线程池类型与应用场景

在java.util.concurrent.Executors类中提供了大量创建连接池的静态方法，常见就有四种。

1. 创建使用固定线程数的线程池

 public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, 
                                      0L, 
                                      TimeUnit.MILLISECONDS, 
                                      new LinkedBlockingQueue<>());
    }

• 核心线程数与最大线程数一样，没有救急线程
• 阻塞队列是LinkedBlockingQueue，最大容量为Integer.MAX_VALUE
• 适用场景：适用于任务量已知，相对耗时的任务
• 案例

/**
 * @author springboot葵花宝典
 * @description: TODO
 */
public class FixedThreadPoolTest {

    static class FixedThreadDemo implements Runnable{
        @Override
        public void run() {
            String name = Thread.currentThread().getName();
            for (int i = 0; i < 2; i++) {
                System.out.println(name + ":" + i);
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        //创建一个固定大小的线程池，核心线程数和最大线程数都是3
        ExecutorService executorService = Executors.newFixedThreadPool(3);

        for (int i = 0; i < 5; i++) {
            executorService.submit(new FixedThreadDemo());
            Thread.sleep(10);
        }

        executorService.shutdown();
    }

}

2.单线程化的线程池，它只会用唯一的工作线程来执行任 务，保证所有任务按照指定顺序(FIFO)执行

public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 
        1, 
        0L,
        TimeUnit.MILLISECONDS, 
        new LinkedBlockingQueue<>()));
    }

• 核心线程数和最大线程数都是1
• 阻塞队列是LinkedBlockingQueue，最大容量为Integer.MAX_VALUE
• 适用场景：适用于按照顺序执行的任务
• 案例

/**
 * @author springboot葵花宝典
 * @description: TODO
 */
public class NewSingleThreadTest {
    static int count = 0;

    static class Demo implements Runnable {
        @Override
        public void run() {
            count++;
            System.out.println(Thread.currentThread().getName() + ":" + count);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        //单个线程池，核心线程数和最大线程数都是1
        ExecutorService exec = Executors.newSingleThreadExecutor();

        for (int i = 0; i < 10; i++) {
            exec.execute(new Demo());
            Thread.sleep(5);
        }
        exec.shutdown();
    }
}

3. 可缓存线程池

public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0,
                                    Integer.MAX_VALUE, 
                                    60L,
                                    TimeUnit.SECONDS, n
                                    ew SynchronousQueue<>());
    }

• 核心线程数为0
• 最大线程数是Integer.MAX_VALUE
• 阻塞队列为SynchronousQueue:不存储元素的阻塞队列，每个插入操作都必须等待一个移出操作
• 适用场景：适合任务数比较密集，但每个任务执行时间较短的情况
• 案例：

/**
 * @author springboot葵花宝典
 * @description: TODO
 */
public class CachedThreadPoolTest {
    static class Demo implements Runnable {
        @Override
        public void run() {
            String name = Thread.currentThread().getName();
            try {
                //修改睡眠时间，模拟线程执行需要花费的时间
                Thread.sleep(100);

                System.out.println(name + "执行完了");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        //创建一个缓存的线程，没有核心线程数，最大线程数为Integer.MAX_VALUE
        ExecutorService exec = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            exec.execute(new Demo());
            Thread.sleep(1);
        }
        exec.shutdown();
    }
}

4.提供了“延迟”和“周期执行”功能的ThreadPoolExecutor

public ScheduledThreadPoolExecutor(int corePoolSize,
                                   ThreadFactory threadFactory,
                                   RejectedExecutionHandler handler{
 super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS, new DelayedWorkQueue(), threadFactory, handler);                     
}

• 适用场景：有定时和延迟执行的任务
• 案例

/**
 * @author springboot葵花宝典
 * @description: TODO
 */
public class ScheduledThreadPoolTest {
    static class Task implements Runnable {
        @Override
        public void run() {
            try {
                String name = Thread.currentThread().getName();

                System.out.println(name + ", 开始：" + new Date());
                Thread.sleep(1000);
                System.out.println(name + ", 结束：" + new Date());

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        //按照周期执行的线程池，核心线程数为2，最大线程数为Integer.MAX_VALUE
        ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(2);
        System.out.println("程序开始：" + new Date());

        /**
         * schedule 提交任务到线程池中
         * 第一个参数：提交的任务
         * 第二个参数：任务执行的延迟时间
         * 第三个参数：时间单位
         */
        scheduledThreadPool.schedule(new Task(), 0, TimeUnit.SECONDS);
        scheduledThreadPool.schedule(new Task(), 1, TimeUnit.SECONDS);
        scheduledThreadPool.schedule(new Task(), 5, TimeUnit.SECONDS);

        Thread.sleep(5000);

        // 关闭线程池
        scheduledThreadPool.shutdown();

    }
}